Moldable glass filled crystalline resin blends offer a high degree of surface hardness, solvent resistance, strength, stiffness, heat resistance, good appearance and colorability. However, polyester formulations typically begin to discolor within days on continuous exposure, in air, to temperatures exceeding about 120.degree. C. The presence of flame retardant additives can cause additional issues to color stability as seen in U.S. Pat. No. 5,371,123 where interaction of certain brominated FR agents and certain glass coatings gave rise to undesirable color.
Hindered phenol antioxidants and phosphites have been used to improve the retention of mechanical properties in polyesters as described in the "Plastics Additives Handbook" R, Gachter and H. Muller, Hanser-Gardner 1996, p92-95. Various additives have been used to improve the retention of polyester viscosity and molecular weigh in the melt. U.S. Pat. No. 5,589,530 to Walsh describes blends of low and high molecular weight polybutylene terephthalate which are melt stabilized by the addition of certain phosphorus-containing compounds. Other additives are described as UV stabilizers, pigments, colorants, fillers, plasticizers, processing aids, antioxidants and the like. Addition of zinc phosphate, sodium acid pyrophosphate, hindered phenols, and pentaerythritol tetrastearate are specifically set forth.
Despite all this work light colored polyester formulations tend to discolor over time, especially when subjected to temperatures exceeding about 120 degrees Centigrade. This discoloration is especially noticeable in polyesters that have white or light color pigmentation. The thermal aging results in a color shift which renders the polyester undesirable for applications such as oven handles or trim, and other appliances subject to heat such as toasters, broilers, dryers, hair styling devices and especially clothing irons. Hence, it is desirable to provide improved polyester resins having desirable properties of strength, stiffness, resistance to deformation at high temperature, as well as good color stability over time when subjected to relatively high temperatures.